The present invention relates to a non-aqueous electrolyte cell including an electrode assembly comprising a strip of negative electrode of light metal as active material, and a strip of positive electrode which are separated from each other by a separator and wound in a spiral, and a specific non-aqueous (organic) electrolyte which is stable with respect to the light metal.
Non-aqueous electrolyte cells including light metal such as lithium as active anode material and oxide or the like as active cathode material have various advantages over other primary cells such as having high voltage, high energy density with low self-discharge, and extremely long storage life, and the range of their applications has been increased with use particularly in advanced electronic appliances.
A typical type of such non-aqueous electrolyte cells has an electrode assembly comprising a strip of negative electrode and a strip of positive electrode which are separated by a separator and wound in a spiral. As shown in FIG. 6, which illustrates a development of the negative electrode in a final discharge state, an anode current collector 8 is connected to the negative electrode 3 close to a point opposite to a winding end 3a of the negative electrode 3. The electrode assembly is wound into a spiral with its negative electrode 3 located outside the positive electrode. The outermost winding of the negative electrode 3 thus has the positive electrode opposed thereto only on its inner side, separated therefrom by the separator. Accordingly, the active material of the negative electrode 3 in its outermost winding reacts with the positive electrode only on one side and is electrochemically consumed about a half as much as compared with other inner windings of the negative electrode 3 where the positive electrode opposes to the negative electrode at both sides. As a result, unreacted active anode material of light metal 3b remains even in the final discharge state as shown in FIG. 6. Particularly, the unreacted active light metal 3b remains in a greater amount between the anode current collector 8 and the winding end 3a of the negative electrode 3 and is partly electrically connected with the anode current collector 8.
When one of a plurality of cells connected in series in their final discharge states is replaced with a new one, the capacity of each cell is unbalanced and the cells are forced to discharge. In that case, since the anode material of active light metal 3b is electrically connected to the anode current collector 8 and still remains in the cell in its final discharge state, a component of active anode material is continuously deposited on the positive electrode by electrolysis, which may occasionally break the separator thus causing a short-circuit between the positive electrode and the negative electrode. Such an internal short-circuit allows a great amount of current to run therein, hence resulting in a sharp increase in temperature. Further, a spark generated when the short-circuit occurs may act as an ignition source and trigger combustion of the cell filled with gas.
The applicant has proposed an improved non-aqueous electrolyte cell capable of overcoming above described disadvantages even when the unreacted active light metal 3b remains in a used battery as disclosed in Japanese Published Unexamined Patent Application No. H5-13089. An arrangement of such a cell is illustrated in FIG. 5. A strip of positive electrode 2, having manganese dioxide as active cathode material and a strip of negative electrode 3, made of a lithium foil, are separated from each other by a separator 4 and wound in a spiral so that the negative electrode 3 comes to an outer side of the positive electrode 2, thus constituting an electrode assembly 1 which is accommodated in a cell housing 9.
In the electrode assembly 1, a winding end 3a of the negative electrode 3 is positioned within an angular range of 180 degrees extending from a winding end 2a of the positive electrode 2 in a direction opposite to a winding direction. In addition, an anode current collector 8 is provided at an inner winding of the negative electrode 3, closer to the spiral core than the end 3a of the negative electrode 3 and radially aligned therewith.
The end 2a of the positive electrode 2 is covered with an insulating tape 10 such as a piece of glass tape. This prevents any burr of the positive electrode 2 produced in cutting the end 2a from piercing the separator 4 to cause leakage.
In the above described non-aqueous electrolyte cell, active anode material of light metal 3b, which remains unreacted mostly in the outermost winding of the negative electrode 3 in the final discharge state, is parted from the anode current collector 8 thus preventing electrical connection there between. An end portion 2b of the positive electrode 2, between its end 2a and the end 3a of the negative electrode 3, is opposed to the negative electrode 3 only at its inner side, with which it reacts intensively, causing a rate controlling reaction in the negative electrode 3 adjacent the end portion 2b of the positive electrode 2. In the case that the cell is in its final discharge state and is forcibly discharged, the rate controlling reaction is further accelerated. Consequently, the remaining component of unreacted active anode material of light metal 3b is cut off by the rate controlling reaction along a one-dotted chain line P shown in FIG. 6, close to the anode current collector 8, and physically separated therefrom. Accordingly, the non-aqueous electrolyte cell of the above described prior art arrangement is capable of avoiding a short-circuit by preventing active anode material from being deposited on the positive electrode 2 by electrolysis even when the cell is forcibly discharged in the final discharge state.
However, the end portion 2b of the positive electrode 2 is specifically set to be as relatively short as 2 to 10 mm. In an actual practice, the electrode assembly 1 is produced on a large scale by winding the positive electrode 2 and the negative electrode 3 in strips separated by the separator 4 and superposed on each other in a spiral with an automatic winder machine. It is thus necessary to determine the overall lengths of the positive and negative electrodes 2, 3 as well as the mechanical precision of the automatic winder machine with extremely high accuracy in order to fabricate an electrode assembly 1 having a positive electrode end portion 2b of appropriate length. Even though these conditions are accurately set, it is hardly attainable to have all the electrode assemblies mechanically mass-produced with the end portion 2b of desired length due to variation in an initial setting at the spiral core or variation in stretch of the negative electrode 3 when wound. Thus it cannot be prevented that an electrode assembly 1 without the end portion 2b of the positive electrode 2 is produced, the end 3a of the negative electrode 3 extending to the end 2a of the positive electrode 2 as shown by a two-dotted chain line in FIG. 5, which results in a diminished yield of products.